This invention concerns inorganic-organic hybrid gels for extraction of chemical species from aqueous solutions.
It is applied in particular to extraction of chemical species composed of either metallic cations such as actinides, lanthanides and transition metals, or anions such as nitrates and sulphates.
It can thus be used in various areas, particularly nuclear, for example for extracting actinides from effluents such as the extracts from processing of irradiated fuels, or to decontaminate radioactive effluents, as well as in the environmental area, for example for depollution of water containing heavy metals or other harmful chemical species.
The processes for extraction of chemical species from aqueous solutions include a large number of solid-liquid processes which use various solid phases such as organic or inorganic resins for extraction through ion exchange, and inorganic or organic supports to which a compound is attached, by grafting or impregnation for example, which has a particular affinity for the chemical species to be extracted.
The document U.S. Pat. No. 4,203,952 (1) describes a process for elimination of heavy metals and transition metals other than platinum from a solution by putting the solution into contact with an inorganic solid phase including on the surface hydroxyl groups on which a silicon compound has been attached by grafting, including thiol or amine-type reactive functions having an affinity for the metal to be extracted.
The compound bound by reaction with the hydroxyl groups on the surface of the substrate has the formula: 
in which Y is an alkoxyl group or a halogen atom capable of reacting with the hydroxyl groups of the substrate and X represents a thiol, amino or nitrated-heterocyclic group.
The technique described in this document involves grafting of the extracting entity on the surface of the substrate which can be silica or a silica gel, but there is no binding of the extracting entity in the internal structure of the substrate.
The document Journal of Radioanalytical and Nuclear Chemistry, Articles, vol. 141, No. 1, 1990, p.107-115(2), describes the use of octyl(phenyl)-N,Nxe2x80x2-diisobutyl carbamoylmethyl phosphine oxide (CMPO) on a solid silica-base support to extract europium and actinides from aqueous solutions. The support with CMPO is prepared from a support made of SiO2 particles. The CMPO extractor is fixed by impregnation of the support using the solvent evaporation technique.
In this case, the extractor is not attached to the substrate by a covalent bond, and it is not integrated by chemical bonding within the structure of the substrate.
This invention precisely involves inorganic-organic hybrid gels in which the extracting entity is integrated in the structure of the gel during its manufacturing.
According to the invention, the inorganic-organic hybrid gel for extraction of at least one chemical species from an aqueous solution includes a network of inorganic units with the formula: 
where M is Si, Ti, Zr or Al, in which organic molecules are integrated which complex the species to be extracted, each organic molecule being covalently bonded to one or several M atoms in the network.
In this gel, the organic molecules include at least one active group which has complexing properties with respect to the species to be extracted.
These molecules are thus chosen depending on the chemical species to be extracted.
If the chemical species to be extracted is a metallic cation such as an actinide and/or a lanthanide, this active group can be chosen from among the amino, ether, hydroxy, amido, pyridino and bipyridino groups, or any other organic group containing an atom which can be an electron donor (O, N, S type).
In the invention gel, the inorganic units can be silica gel, titanium oxide gel, zirconium oxide gel or alumina gel.
According to a preferred embodiment of the invention, the hybrid gel is silica-based and the inorganic units of the network are: 
According to a first embodiment of the invention, in the organic-inorganic hybrid gel, all of the M atoms of the network are bonded to a complexing organic molecule.
A gel of this type is particularly valuable because it includes a very high density of complexing molecules having an affinity for the chemical species to be extracted.
According to a second embodiment of the invention gel, only some of the M atoms of the network are covalently bonded to a complexing organic molecule. In this case, at least 9% of the M atoms of the network are linked to complexing molecules.
The complexing molecules could, for example, be based on the following formulae: 
The invention also involved a process for preparing an organic-inorganic hybrid gel as defined above, from silicon alkoxides functionalised with organic groups and possibly silicon alkoxides. In this process, the sol-gel technique is used and, in the presence of water, at least one functionalised metallic alkoxide
LM(OR1)l(I) or (R1O)l MLM(OR1)lxe2x80x83xe2x80x83(II)
in which M represents Si, Ti, Zr or Al
L represents an organic group with at least one organic complexing group;
R1 represents an organic group, preferably an alkyl group; and
l=3 when M represents Si, Ti or Zr; or
l=2 when M represents Al.
with the following formula is polymerised by polycondensation: According to a variant of the application of the invention process, the functionalised metallic alkoxide corresponding to formula (I) or (II) described above is polymerised with a metallic alkoxide with the formula:
M(OR2)mxe2x80x83xe2x80x83(III)
in which M represents Si, Ti, Zr or Al, R2 is an organic group, preferably an alkyl group, and m=4 when M represents Si, Ti or Zr or m=3 when M represents Al.
The alkyl groups used for R1 and R2 preferably have 1 to 12 carbon atoms. In particular, methyl and ethyl groups can be used.
According to the invention, the organic complexing function(s) present in the organic group L can be in particular amino, ether, hydroxy, amido, pyridino and bipyridino groups.
Such groups L may be based on one of the following formulae for example: 
The functionalised metallic alkoxides which could be used in the invention process include, for example, those corresponding to the following formulae: 
in which R is an organic group, preferably an alkyl group, having for example 1 to 12 carbon atoms, such as methyl and ethyl groups.
When the hybrid gels are obtained solely from functionalised silicon alkoxides, the gel network is formed from the following organic-inorganic units: 
If the hybrid gel is obtained from functionalised metallic alkoxides and metallic alkoxides, the formation of the gel corresponds to the following schemes;
nM(OR2)m+LM(OR1)lxe2x86x92[LMOy, nMOm]
or
nM(OR2)m+(R1O)l MLM(OR1)lxe2x86x92[LM2Oz, nMOm]
in which M, R1, R2, l, and m are as defined above, n is a whole number from 1 to 1000 and y and z are numbers from 1 to 4.
The resulting material is a hybrid gel of the metallic oxide M in which the organic molecules L with extracting properties are integrated by covalent bonding during the formation of the solid phase of the gel.
This mode of manufacturing differs from classic grafting processes in that the mineral support, i.e. the gel, is formed at the same time that the organic unit is integrated into the material. The stoichiometry and the molecular structure of the solid can thus be precisely controlled. By appropriately choosing the organic group L and the mineral skeleton, certain properties of the gel obtained can be varied, such as the selectivity with respect to chemical species to be extracted such as actinides, and the three-dimensional molecular structure of the material.
The following formulae are examples of hybrid silica gels in accordance with the invention: 
The formulae given above for the gels of the invention and their preparation correspond to a totally condensed three-dimensional network. The gels obtained generally have a degree of condensation of 70 to 90% for which there are some OH or OR groups which are non-condensed. LSiO1.5 becomes, in this case, LSiOx(OH)y(OR)z with 1xe2x89xa6xxe2x89xa61.5 and 0xe2x89xa6y+zxe2x89xa60.5.
According to another embodiment of the invention, another chemical structure based on an appropriate alkoxide can be integrated into the structure of the gel. For example, a structure based on the formula (R3O)l Mxe2x80x94Xxe2x80x94M(OR3)l (IV), in which M and l are as defined above, R3 is an organic group, preferably an alkyl group having for example 1 to 12 carbon atoms, and X represents an organic group which is non-complexing with respect to the species to be extracted, allowing for variation in the spatial arrangement properties of the gel in order to obtain other results.
Organic groups of this type include aromatic or aliphatic groups such as those corresponding to the following formulae: 
In applying the process for preparation of a hybrid gel according to the invention, the metallic alkoxide(s) are generally dissolved in an appropriate solvent such as ethanol, then water and a nucleophile catalyst such as ammonium fluoride:are added. Basic or acidic catalysts can also be used.
The quantities of solvent are generally such that they correspond to 0.5 to 1 ml of ethanol per mmole of silicon and the quantity of water added must correspond to at least 0.5 equivalents of water per alkoxyl group OR1 and possibly OR2 and OR3 present in the metallic alkoxides used. If the metal M is silicon, the quantity of catalyst is generally 0.1% with respect to silicon.
The reaction can be carried out at room temperature, with stirring, and then letting the reaction medium sit until gelation and then leaving it to age for a week, for example.
To improve the performance of the gel obtained from this process, the gel can be made in the presence of the chemical species to be extracted in order to include this species in the gel during its manufacturing and then subjecting the gel to washing in order to eliminate this species. By an imprint effect on the molecular structure or the texture (porosity, specific surface area) of the gel (template effect), this procedure improves its selectivity with respect to the species to be extracted. This species could in particular be a metallic ion.
The invention also involves the functionalised silicon alkoxides used for preparation of the hybrid silica gels, these alkoxides being based on the following formulae: 
The gels obtained with the invention process can be in granular or powder form, or in the form of thin layers or membranes.
They can be used in an process for extraction of ions present in an aqueous solution by putting this solution into contact with the gel, followed by separation of the gel which has bound the ions to be extracted. For example, the ions to be extracted could be actinide and/or lanthanide ions. When a gel in granular form is used, the extraction process corresponds to an extraction chromatography and it can be done in a column filled with granular gel.
If the gel is in thin layer form, the extraction can be done by a membrane separation technique by putting in circulation on one side of the membrane the aqueous solution containing the ions to be extracted and on the other side of the membrane, an aqueous re-extraction solution.